This invention relates to an air/fuel ratio control system for controlling the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to means for determining the timing of initiation of the air/fuel ratio control which is performed by such air/fuel ratio control system, in dependence on the engine temperature, etc.
An air/fuel ratio control system for use with an internal combustion engine has already been proposed by the applicants of the present application, which comprises an O.sub.2 sensor provided in the exhaust system of the engine for detecting the oxygen concentration in the engine exhaust gases, an air/fuel ratio control valve having its valve body position disposed to determine the air/fuel ratio of an air/fuel mixture being supplied to the engine, an actuator arranged to drive the air/fuel ratio control valve in response to an output signal generated by the O.sub.2 sensor, and an engine coolant temperature sensor arranged to detect the temperature of the engine coolant.
The O.sub.2 sensor, which is comprised of a sensor element made of stabilized zirconium oxide or a like material, is adapted to detect the concentration of oxygen in the engine exhaust gases in such a manner that the output voltage of the O.sub.2 sensor varies correspondingly to a change in the conduction rate of oxygen ions through the interior of the zirconium oxide or the like material which change corresponds to a change in the difference between the oxygen partial pressure of the air and the equilibrium partial pressure of the oxygen in the engine exhaust gases. Further, the O.sub.2 sensor has its internal resistance also variable with a change in the degree of activation of the sensor. Therefore, if the O.sub.2 sensor is arranged with its one terminal connected to a power supply by way of a resistance and its other or opposite terminal grounded, the potential at the junction of the resistance with the O.sub.2 sensor, that is, the output voltage of the O.sub.2 sensor decreases as the activation of the O.sub.2 sensor proceeds.
Therefore, according to the aforementioned proposed air/fuel ratio control system, the air/fuel ratio feedback control operation is initiated only after the O.sub.2 sensor has been fully activated, that is, upon the lapse of a predetermined period of time after the output voltage of the O.sub.2 sensor has dropped below a predetermined value.
On the other hand, an internal combustion engine in general is provided with a choke valve arranged at an air intake of the carburetor for closing and opening the same air intake in order to supply a rich mixture to the engine at the start of the engine under a low temperature condition. If the choke valve is of the type being automatically opened or closed in response to a change in the engine temperature, it is closed to cause supply of a rich mixture to the engine at the start of the engine when the engine temperature is low. If the air/fuel ratio feedback control operation is carried out on this occasion, the actuator which drives the air/fuel ratio control valve is driven toward the LEAN side where the air/fuel ratio is large, so that the air/fuel ratio of the mixture being supplied to the engine has a value approximate to the thereotical air/fuel ratio. That is, the choke valve cannot exhibit its proper function.
Particularly in a choke system in which the choke valve is controlled for opening or closing in immediate response to the engine coolant or cooling water temperature or by means of an electric heater or the like which has a heating temperature characteristic equivalent to the engine coolant temperature, in very cold weather there is the possibility that the engine coolant or cooling water temperature does not rise up to a value at which the choke valve is opened even after the completion of activation of the O.sub.2 sensor which has rapidly been heated to a sufficiently high temperature by the exhaust gases in the exhaust system of the engine, due to the fact that the increase rate of the engine coolant temperature is much smaller than that of the temperature of the O.sub.2 sensor. As a consequence, the air/fuel ratio feedback control operation is initiated with the choke valve still closed, resulting in the aforementioned disadvantage.